Synthesis and reactivity of low-valent amido, imido, azavinylidene, and nitrido complexes of tungsten

Article abstract of DOI:10.1021/om00017a048

Amido complexes Tp′(CO)₂W(NHR) [Tp′ = hydridotris(3,5-dimethylpyrazolyl)borate; R = Bu^t, 1a; R = Ph, 1b; R = Buⁿ, 1c; R = CH₂Ph, 1d; R = H, 1e] have been synthesized by reaction of the corresponding amine (NH₂R) with Tp′(CO)₂WI (2). These ambiphilic amido complexes have been utilized as precursors to both anionic imido complexes, Tp′(CO)₂W(NR)^- (R = Ph, 3a; R = CH₂Ph, 3b; R = H, 3c), and cationic imido complexes, Tp′(CO)₂W(NR)⁺ (R = Bu^t, 5a; R = Ph, 5b; R = Buⁿ, 5c; R = CH₂Ph, 5d; R = H, 6a; R = CPh₃, 6b). Treatment of the unstable anionic imido species 3a-c with PhCH₂Br (3a) or MeI (3b,c) leads to the formation of dialkyl substituted amido complexes Tp′(CO)₂W(N(R)R′) (R = Ph, R′ = CH₂Ph, 4a; R = CH₂Ph, R′ = Me, 4b; R = R′ = Me, 4c). The structures of 4a and 4c have been confirmed by X-ray diffraction. (4a·CH₂Cl₂ crystallizes in space group P1 with Z = 2, a = 11.452(2) A?, b = 11.597(2) A?, c = 14.144(2) A?, α = 86.78(1)°, β = 84.41(1)°, γ = 64.37(1)°, R = 3.7%, R_w = 4.6%; 4c crystallizes in space group P1 with Z = 2, a = 11.151(4) A?, b = 11.702(3) A?, c = 10.391(4) A?, α = 93.46(3)°, β = 108.56(3)°, γ = 63.20(3)°, R = 3.9%, R_w = 4.3%.) Reactivity studies were carried out with the cationic nitrene complexes, Tp′(CO)₂W(NR)⁺. Deprotonation of Tp′-(CO)₂W(NCH₂R)⁺ (R = Prⁿ, 5c; R = Ph, 5d) yields azavinylidene complexes, Tp′(CO)₂W-(N=CHR) (R = Prⁿ, 7a; R = Ph, 7b). The barrier to rotation around the WNC unit of 7a as determined by variable temperature NMR studies is 9.6 kcal/mol. Addition of LiBH₄ to the cationic nitrene complex 5b results in formation of a formyl intermediate, Tp′(CO)(CHO)W-(NPh) (8), which undergoes hydride migration from carbon to nitrogen to form the amido complex 1b at -70°C. Deprotonation of the parent nitrene cation Tp′(CO)₂W(NH)⁺ (6a) or reaction of [(Ph₃P)₂N][N₃] with Tp′(CO)₂WI (2) yields an unstable nitrido complex Tp′(CO)₂WN (9). Complex 9 reacts with a variety of electrophiles (RX) to form both dicarbonyl, [Tp′-(CO)₂W(NR)][X] (R = H, X = BF₄^-, 6a; R = CPh₃, X = PF₆^-, 6b; R = Me, X = OTf^-, 10), as well as monocarbonyl, Tp′(CO)(X)W(NR) (R = Ts, X = Cl, 11a; R = C(O)CH₃, X = Cl, 11b; R = C(O)CH₃, X = OC(O)CH₃, 11c), imido products. The X-ray structures of nitrene complexes Tp′(CO)₂W(NPh)⁺ (5b) and Tp′(CO)ClW(NTs) (11a) reveal linear imido ligands for both cationic and neutral complexes with WN bond lengths of 1.775(7) and 1.78(1) A?, respectively. (5b crystallizes in space group P2₁/n with Z = 4, a = 10.301(2) A?, b = 10.389(2) A?, c = 27.816(7) A?, β = 97.69(2)°, R = 3.6%, R_w = 4.5%; 11a crystallizes in space group P2₁ with Z = 2, a = 8.222(3) A?, b = 17.256(6) A?, c = 9.561(4) A?, β = 100.85(3)°, R = 4.0%, R_w = 4.6%.)